Ceramics have been receiving increasing attention for use in structural components. However, physical properties such as flexural strength or toughness are not always at the levels necessary in modern structural uses.
It has been noted that these properties often increase with increasing density of the ceramic components and attention is focusing on the production of ceramic products with densities approaching the theoretical density. Submicron ceramic powders are desirable because of their ability to give near theoretical density ceramic bodies upon sintering. High density ceramic bodies formed from submicron powders exhibit the superior mechanical strength and toughness required for ceramic materials used in severe environmental conditions.
Ceramic particles have long been obtained by ball milling or similar comminution methods. Such methods often provide wide particle size distribution and are extremely time and energy-consuming.
For the preparation of bimetal oxide ceramics such as perovskite materials, or solid solution phases such as Y.sub.2 O.sub.3 /ZxO.sub.2, mixtures of these individual oxide powders are heated to high temperature to effect the formation of the bimetal oxide phase by solid state reactions. These solid state reactions are slow and require several heat-and-grind operations to effect homogeneity. Such operations are therefore expensive and introduce contaminants during the grinding processes. Contaminants degrade the mechanical properties of the final ceramic.
A much better method of making fine-grain and high purity ceramic oxide powders is via the hydrolysis of inorganic salts, acetates, oxycarbonates, oxalates, alkoxides etc. Hydrous oxides are prepared from an aqueous solution of a metal salt (or mixture of metal salts) precursor. The quality of the hydrolysis depends upon reaction conditions such as temperature, concentration, pH and mixing conditions. On the other hand, the quality of the ceramic oxide produced depends on the method of drying the solid. When the hydrous oxide sol is converted into a gel which is then dried at near-ambient conditions, the product is usually highly agglomerated as a result of the compressive forces applied by the liquid's surface tension. Such product materials must then be crushed to smaller grain size and do not produce useful dense ceramic materials.
U.S. Pat. No. 4,314,827 discloses a process for preparing a high density ceramic from a gel precursor by use of a sintering technique which collapses the gel into a more dense product. The gel-formed product is a continuous alpha alumina phase with a secondary stabilizing phase of either zirconia, an alumina-zirconia spinel or preferably mixture of zirconia and the spinel. The process involves the conversion of a stable sol or colloidal dispersion into a gel by drying an aqueous gel overnight at a temperature of approximately 90.degree. C. The gel is then crushed to yield a desired product size of 0.5 millimeter or below. The crushed ceramic is then sintered to yield the desired high density ceramic for use as an abrasive. However, the ceramic has limited use due to the relatively large size of its particles.
U.S. Pat. No. 4,429,051 discloses the production of an alumina/zirconia ceramic from a sol precursor. The sol is produced, for example, by hydrolysis of zirconia/hydroxide which is admixed with an alpha sol or slurry followed by spray drying The spray drying step produces either a gel of zirconia alumina oxide or alumina oxide particles bonded together by zirconia oxide gel spheres depending on whether the alumina oxide is added to a zirconia sol as a sol or a powder respectively. This product is then further dried to yield a powder which is then milled, calcined and remilled to provide a relatively defined particle size product which provides a sintered ceramic with a relatively high fracture toughness.
It has additionally been disclosed in U.S. Pat. No. 3,637,407 to prepare pure alpha alumina from an alumina precursor powder prepared by vapor phase hydrolysis of aluminum isopropoxide. The patent discloses that the crystalline size of the alpha alumina is in the range of 0.5 to 1 micron which agglomerates into much larger clumps. The agglomerates require conventional, long time ball milling to reduce the particle size to yield a ceramic powder which upon sintering will exhibit high fracture toughness. Despite the ball milling, the resulting body has a density much less than theoretical density.
Spray drying has also been proposed, for an example, drying ceramic glass slurries in U.S. Pat. No. 4,552,852. In that process, fine particle size was obtained by directly wet milling the ceramic oxides for a period up to 10 hours or longer. Afterwards the wet milled slurry was spray dried.
It has also been proposed to produce silica gel by a continuous process, such as that disclosed by U.S. Pat. No. 2,868,280 to Sargent et al. The Sargent et al process requires special customized equipment to prevent pluging by the particulate material and has not been employed for production of fine particulate ceramics.
Japanese patent application No. 54-25523 discloses the production of fine grain zirconia ceramics by coprecipitating zirconia sols in an aqueous solution admixed with various optional stabilizing agents. Ammonia is the precipitating agent. The precipitate is filtered and resuspended in an organic solvent where it is made anhydrous by azeotropic distillation followed by drying to yield a stabilized zirconia powder with reported sintered bulk densities of 5.2 to 5.5 g/cm.sup.3. Azeotropic distillation is also disclosed in U.S Patent No. 4,365,011 to Bernard et al in which a sinterable zirconia percipitated directly from an alcohol solution with ammonium is washed with a hydrophilic solution and subjected to a drying azeotropic distillation with a solvent such as benzene which displaces the residual water.
U.S. Pat. No. 4,501,818 discloses an alcohol precipitation of zirconia sols in anhydrous ethanol with an alkaline metal hydroxide. The resulting precipitate is filtered, dried, washed with water, and dried again. The process allegedly reduced the filter clogging problems of Bernard while obtaining a zirconia fired density of 5.75 to 5.99 Mg/M.sup.3. This process requires removal of the residual alkali metal (e.g., sodium) by either washing or precipitation in order to obtain a pure powder. This process is also burdened with the problem that a settled precipitate is formed as opposed to a sol, which is purposely avoided and involves multiple drying steps in a rather complicated and expensive overall process of limited applicability.
A majority of the proposed methods thus require extensive ball milling or complicated chemical processes to achieve the desired particle size. Those processes which propose using a ceramic sol generally require that the sol be transformed into a gel for further processing into a particulate powder. The art still awaits a simple process for transforming a sol directly and simply into a fine particulate ceramic powder or powder mixture which will yield high density ceramics upon sintering. Despite the activity in the art, there is also a gap in providing a process for producing a fine particulate ceramic powder directly from a sol or solution of the ceramic oxide which ceramic powder has a high pore volume, a high surface area and is able to achieve densities approaching that of theoretical upon sintering.